1. In a saucepan, pour 5 cups of milk and place it on the heat to boil. In a bowl, put the sugar and the orange zest. Mix well so the orange releases its essential oil and flavors the sugar. Add the remaining milk, the custard powder, the sugar, the cocoa, and mix until the mixture starts to thicken. Add the finely chopped couverture chocolate and the butter, if using. Stir for 1-2 minutes and remove from the heat.
2. In a bowl, mix the milk and the cognac. Dip the biscuits in the mixture and arrange them on the bottom of a rectangular baking dish. Spread 1/3 of the cream over them. Spread another layer of cream and add another 1/3 of the cream. Arrange another row of biscuits and pour the remaining cream on top.
3. Let the cream cool slightly for 3-4 minutes and sprinkle with the coarsely chopped walnuts.
